U.S. patent application number 10/084765 was filed with the patent office on 2002-08-22 for stents with temporary retaining bands.
This patent application is currently assigned to SCIMED LIFE SYSTEMS, INC.. Invention is credited to Kocur, Gordon J..
Application Number | 20020116050 10/084765 |
Document ID | / |
Family ID | 22872957 |
Filed Date | 2002-08-22 |
United States Patent
Application |
20020116050 |
Kind Code |
A1 |
Kocur, Gordon J. |
August 22, 2002 |
Stents with temporary retaining bands
Abstract
A self-expanding stent is formed of a stent framework and
retaining segments disposed about the framework. The retaining
segments hold the stent in a partially or fully contracted state in
a bodily vessel. The retaining segments are constructed to fail
after a predetermined time or upon the application of a
predetermined force. Upon failure of the segments, the stent
expands in the bodily vessel.
Inventors: |
Kocur, Gordon J.; (Lino
Lakes, MN) |
Correspondence
Address: |
VIDAS, ARRETT & STEINKRAUS, P.A.
6109 BLUE CIRCLE DRIVE
SUITE 2000
MINNETONKA
MN
55343-9185
US
|
Assignee: |
SCIMED LIFE SYSTEMS, INC.
Maple Grove
MN
55311-1566
|
Family ID: |
22872957 |
Appl. No.: |
10/084765 |
Filed: |
February 26, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10084765 |
Feb 26, 2002 |
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09232404 |
Jan 15, 1999 |
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6350277 |
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Current U.S.
Class: |
623/1.15 |
Current CPC
Class: |
A61F 2002/91558
20130101; A61F 2/91 20130101; A61F 2/90 20130101; A61F 2/915
20130101; A61F 2250/0071 20130101; A61F 2002/9511 20130101; A61F
2210/0004 20130101; A61F 2002/91541 20130101 |
Class at
Publication: |
623/1.15 |
International
Class: |
A61F 002/06 |
Claims
1. An expandable stent comprising: an expandable stent framework,
the stent framework expandable from a reduced diameter
configuration to a fully expanded configuration; and at least one
stent retaining segment disposed about the stent, the stent
retaining segment maintaining the stent framework in a less than
fully expanded configuration, the stent retaining segment
constructed and arranged to fail upon degradation of at least a
portion of the segment.
2. The stent of claim 1 comprising a plurality of stent retaining
segments.
3. The stent of claim 1 wherein the retaining segment is formed of
at least one segment of inert material and at least one segment of
biodegradable material joined together.
4. The stent of claim 3 wherein the retaining segment comprises a
plurality of alternating inert segments and biodegradable segments
joined together.
5. The stent of claim 1 wherein the retaining segment is made of a
biodegradable material.
6. The stent of claim 5 wherein the biodegradable material is
provided with a treatment agent.
7. The stent of claim 1 wherein the retaining segment is provided
with a treatment agent.
8. The stent of claim 1 wherein the retaining segment is disposed
on the outside of the stent framework.
9. The stent of claim 1 wherein the retaining segment is interwoven
through the stent framework.
10. The stent of claim 1 wherein the retaining segment is
constructed to fail after a predetermined period of time in the
body.
11. The stent of claim 10 wherein the period of time is at least
one month.
12. The stent of claim 10 wherein the retaining segment is in the
form of a band disposed about the stent.
13. The stent of claim 1 wherein the stent framework comprises a
plurality of interconnected struts and the retaining segment
interconnects connects no more than two adjacent struts.
14. The stent of claim 1 wherein the retaining segment is in the
form of a web.
15. An expandable stent comprising: an expandable stent framework,
the stent framework expandable from a reduced diameter
configuration to a fully expanded configuration; and at least one
stent retaining segment disposed about the stent, the stent
retaining segment maintaining the stent framework in a less than
fully expanded configuration, the stent retaining segment
constructed and arranged to have at least one fatigue point
thereon.
16. The stent of claim 15 the retaining segment having a plurality
of fatigue points thereon.
17. The stent of claim 15 wherein the retaining segment narrows at
the fatigue point.
18. The stent of claim 15 wherein the retaining segment is made of
an inert, material.
19. The stent of claim 18 wherein the material is PTFE.
20. The stent of claim 15 wherein the stent retaining segment is
constructed to fail after a predetermined amount of time in a
bodily lumen.
21. The stent of claim 15 wherein the retaining segment is
constructed to fail after at least one month in a bodily lumen.
22. The stent of claim 15 where the stent retaining segment is
constructed to fail upon the application thereto of a predetermined
force.
23. A treatment method comprising the steps of: i) implanting an
expandable prosthesis in a bodily lumen, the expandable prosthesis
comprising 1) an expandable prosthesis framework, the prosthesis
framework expandable from a reduced diameter configuration to a
fully expanded configuration; and 2) a prosthesis retaining segment
which is constructed to fail upon dissolution of at least a portion
thereof, the segment formed at least in part of a biodegradable
material the prosthesis retaining segment disposed about the
prosthesis and maintaining the prosthesis framework in a reduced
diameter configuration.
24. The method of claim 23 wherein the prosthesis retaining segment
is constructed to fail after a predetermined time in a bodily
lumen.
25. The method of claim 23 wherein the predetermined time is at
least one month.
26. A treatment method comprising the steps of: i) implanting an
expandable prosthesis in a bodily lumen, the expandable prosthesis
comprising 1) an expandable prosthesis framework, the prosthesis
framework expandable from a reduced diameter configuration to a
fully expanded configuration; and 2) a prosthesis retaining segment
which is constructed to have at least one fatigue point thereon to
facilitate failure of the segment under a predetermined set of
conditions, the prosthesis retaining segment disposed about the
prosthesis and maintaining the prosthesis framework in a reduced
diameter.
27. An expandable medical endoprosthesis for implantation in a
bodily vessel comprising: an expandable prosthesis framework
expandable from a reduced diameter configuration to a fully
expanded configuration; and at least one endoprosthesis retaining
structure attached to the endoprosthesis framework and disposed
about the endoprosthesis framework, the endoprosthesis retaining
structure maintaining the endoprosthesis framework in a reduced
diameter configuration.
28. The endoprosthesis of claim 27 wherein the retaining structure
is in the form of a segment.
29. The endoprosthesis of claim 27 wherein the retaining structure
is in the form of a web.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates specifically to stents but also
generally to any expandable, implantable endoprosthesis such as
grafts, stent-grafts and the like. When the term `stent` is used
herein, it is to be understood in a general sense as including all
such expandable prostheses, unless otherwise indicated.
[0002] The use of endoprostheses such as stents, stent-grafts and
grafts is well known in maintaining the patency of bodily vessels
including blood vessels and biliary ducts. Typically, an
endoprosthesis is implanted in a vessel which has been occluded,
which is subject to an aneurysm, which has a lesion present or is
otherwise damaged. Often, during the implantation of the
endoprosthesis, the vessel will suffer from trauma. The trauma may
be as a result of the dilation prior to the implantation of the
endoprosthesis, the presence of a foreign body (the endoprosthesis)
in the bodily vessel or as a result of other causes. Regardless of
the source of the trauma, the vessel may be in a weakened and
inflamed state as a result of implantation of the endoprosthesis.
Although it is desirable to maintain the vessel at as large a
diameter as possible to minimize the possibility of restenosis, the
weakening of the vessel resulting from the trauma limits the extent
to which the vessel can be dilated.
[0003] The endoprostheses that are currently available are
typically balloon expandable, self-expanding, or balloon assisted
self-expanding devices. Balloon expandable stents achieve their
maximum diameter upon expansion by a balloon. Once they have been
seated in the vessel, they are incapable of further expansion
unless a balloon is reinserted in the stent and expanded.
Self-expanding and balloon assisted expandable stents, on the other
hand, continually exert an outward force as they try to attain
their maximum possible diameter. Thus, even after the stent is
implanted, if it has not reached its maximum diameter it continues
to try to open further exerting a force on the vessel walls.
[0004] It would be desirable to provide an endoprosthesis which has
some of the characteristics of balloon expandable stents following
deployment and which has some of the properties of self-expanding
stents after a predetermined period of time or after the
application of a predetermined amount of force thereto. In
particular, it is desirable to provide an endoprosthesis which does
not impart to the vessel walls the outward forces associated with a
self-expanding stent while the vessel is recovering from the trauma
of the deployment procedure and yet provides the outward expanding
force of a self-expanding stent when the vessel is sufficiently
recovered from the trauma.
[0005] It is also desirable to provide an endoprosthesis which only
partially expands or does not expand at all during deployment and
which expands at some later point following deployment.
[0006] The present invention provides such an endoprosthesis.
[0007] For the purposes of this disclosure, unless otherwise
indicated, the term `degradation` shall refer to degradation in its
ordinary sense as well as biodegradation, erosion, and
dissolution.
BRIEF SUMMARY OF THE INVENTION
[0008] The invention is directed to an expandable stent comprising
an expandable stent framework which is expandable from a reduced
diameter configuration to a fully expanded configuration. At least
one stent retaining segment which is constructed to fail is
disposed about the stent framework to maintain the stent framework
in a reduced diameter state until the retaining segment fails. The
retaining segment may be constructed to fail after a predetermined
period of time has elapsed and/or after a predetermined amount of
force has been applied to the stent. The retaining segment may be
entirely biodegradable, biodegradable in part, erodible or made of
an inert material with fatigue points.
[0009] In one embodiment of the invention, the retaining segment
forms a band around the outside of the stent or interwoven about
the stent. In particular, the retaining segment may be woven such
that it alternates on the inner and outer strut surfaces.
[0010] The retaining segment may also be provided in the form of a
web.
[0011] The invention is also directed to treatment methods in which
the inventive stent is deployed and further expands upon failure of
the retaining segments. In the case of an inert segment, failure of
the retaining segment may result from mechanical failure. Stresses
on the stent which are likely to result in or contribute to the
desired failure include pulsing blood pressure which is thought to
cause an alternating lengthening and shortening of the stent.
[0012] In the case of a biodegradable segment or an erodible
segment, failure results from degradation or erosion of the
segment. Finally, in the case of a segment made of inert material
and connecting dots of biodegradable material, failure results from
degradation or erosion of the connecting dots.
[0013] More generally, the invention is directed to the application
of a secondary increased expansive force to a body lumen by an
implantable expandable medical device such as a stent
endoprostheses including stents, stent-grafts, grafts and vena cava
filters at a predetermined time following implantation into a body
lumen.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0014] FIG. 1 shows a perspective view of an inventive stent.
[0015] FIG. 2 shows a perspective view of another inventive
stent.
[0016] FIG. 3 shows a perspective view of an inventive stent
illustrating several types of retaining segments.
[0017] FIG. 4a shows a perspective view of another inventive
stent.
[0018] FIG. 4b shows a perspective view of another inventive
stent.
[0019] FIG. 4c shows a cross-sectional view of the stent of FIG. 4b
along the lines 4c-4c.
[0020] FIGS. 5a-c show retaining segments with fatigue points.
[0021] FIG. 6a shows a perspective view of a retaining segment.
[0022] FIG. 6b shows a perspective view of another retaining
segment.
[0023] FIGS. 7 shows a perspective view of another embodiment of
the inventive stent.
[0024] FIG. 8 shows a perspective view of another embodiment of the
inventive stent.
[0025] FIG. 9 shows a partial cross-section of a stent with a
retaining segment fused to the stent.
[0026] FIG. 10 shows a partial cross-section of a stent with a
retaining segment bonded the stent.
[0027] FIGS. 11a-e are schematic representations of stent with
retaining segments in the form of webs.
[0028] FIG. 12 is a perspective view of a die for use in the
present invention.
[0029] FIGS. 13a-d are schematic representations of a stent with a
retaining band prior to (FIGS. 13a and 13c) and following (FIGS.
13b and 13d) failure of the retaining segment.
DETAILED DESCRIPTION OF THE INVENTION
[0030] While this invention may be embodied in many different
forms, there are described in detail herein specific preferred
embodiments of the invention. This description is an
exemplification of the principles of the invention and is not
intended to limit the invention to the particular embodiments
illustrated.
[0031] With reference to the Figures in general and FIG. 1 in
particular, the present invention is directed to an expandable
stent, shown generally at 100, comprising an expandable stent
framework 110, and a stent retaining segments 115 which are
selected portions of the stent framework. Expandable stent
framework 110 is a self-expanding stent which is expandable from a
reduced diameter configuration to a fully expanded configuration.
Stent retaining segments 115 maintains the stent framework in a
less than fully expanded configuration until it fails. In the
embodiment of FIG. 1, one pair of struts 120 per stent section 125
are joined together with a retaining segment 115. Each stent
retaining segment 115 is circumferentially offset from the adjacent
stent retaining segment by one strut 120.
[0032] The invention also contemplates providing a plurality of
stent retaining segments per stent section. FIG. 2 shows stent 100
with two stent retaining segments 115 per stent section 125. Stent
retaining segments 115 are oppositely disposed from one another
across stent section 125.
[0033] FIG. 3 illustrates several other configurations of stent
retaining segments. In stent section 125a, stent retaining segments
115a join every other pair of adjacent struts 120 in the center of
the struts. In stent section 125b, every pair of adjacent struts
are joined by stent retaining segments 115b. Stent retaining
segments 115b are seen to alternate between a first end of adjacent
and a second end of adjacent struts. The stent of FIG. 3 also shows
a retaining segment 115d connecting adjacent stent sections 125d
and 125e. The stent of FIG. 3 also shows a retaining segment 115e
extending between three adjacent struts. It is understood that the
stent of FIG. 3 is intended to be illustrative of several
embodiments. Those of ordinary skill in the art will recognize that
the invention contemplates stents in which a desired number of
stent sections have stent retaining segments as in section 125a.
Similarly, the invention contemplates stents having stent retaining
segments arranged entirely as in section 125b. It will also be
recognized that the invention contemplates stents with
interconnected stent sections as in sections 125d and 125e.
Finally, it will be recognized that each retaining segment may
restrain multiple struts beyond the two shown in the figure.
[0034] The use of small retaining segments is beneficial for
several reasons. First, retaining individual segments of the stent
allows for greater stent flexibility. Second, using relatively
small retaining segments results in a reduced negative tissue
response to the implant and the segment in particular. Third,
retaining only stent segments allows for better control over stent
expansion and thus, how the additional secondary force is applied
to the body lumen.
[0035] The invention also contemplates the use of larger stent
retaining segments in the form of stent retaining bands. In one
embodiment of the invention, the stent retaining segments form one
or more bands extending around the circumference. The stent of FIG.
4a, shown generally at 100, comprises an expandable stent framework
110, and a stent retaining segment 115 which is disposed about the
circumference of the stent 110 in the form of a band. Expandable
stent framework 110 is a self-expanding stent which is expandable
from a reduced diameter configuration to a fully expanded
configuration. Stent retaining segment 115 maintains the stent
framework in a less than fully expanded configuration until it
fails. The invention also contemplates the possibility of providing
the stent with additional retaining bands beyond the two shown in
FIG. 4a.
[0036] The band-like segments may extend around the stent from the
outside of the stent or may be interwoven around the circumference
of the stent as shown in FIGS. 4b and 4c. Bands 115 weave
in-between struts 120.
[0037] The invention contemplates several different types of stent
retaining segments. One stent retaining segment suitable for use in
the present invention is shown in Fig. 5a. Stent retaining segment
215 has a single fatigue point 220. Fatigue point 220 is a narrowed
region of the band. FIG. 5b shows a stent retaining band 215 with
two fatigue points 220. The shape of the fatigue points shown in
FIGS. 5a and 5b is intended to be for illustrative purpose only.
Fatigue points of other geometries and configurations may also be
used. A retaining band with a single fatigue point may also be used
as may a retaining band with additional fatigue points. FIG. 5c
shows a retaining segment 215 with two holes 223 punched
therethrough. Fewer or additional holes may also be present in the
region to weaken the segment.
[0038] Retaining band 215 is made of an inert or biostable material
such as polytetrafluoroethylene (PTFE). Other inert or biostable
materials may also be used. Desirably, a material with a low tissue
response such as PTFE, polyurethanes, polyesters, and silicones
will be used.
[0039] Another suitable stent retaining segment is shown generally
at 315 in FIG. 6a. Retaining segment 315 is formed of two segments
of inert or biostable material 325 and a segment of biodegradable
material 320 joined together. Such a design may also be used for a
stent retaining band as shown in FIG. 6b. Retaining band 315 is
formed of a segment of inert or biostable material 325 and a
segment of biodegradable material 320 joined together. As shown in
FIGS. 6a and 6b, biodegradable segment 320 is provided in the form
of a dot. Biodegradable segment 320 may suitably be provided in
other shapes as well. In particular, the retaining segment or band
may be provided with the same general shape as bands 215 shown in
FIGS. 5a and 5b, with the fatigue point replaced by a biodegradable
segment where the band narrows. An embodiment of retaining band 315
having two inert segments 325 and two biodegradable segments 320 is
shown in FIG. 6c. A retaining segment may be similarly prepared
with multiple biodegradable segments. The invention further
contemplates the use of additional inert segments and additional
biodegradable segments.
[0040] Another embodiment of the invention is shown in FIG. 7.
Stent 100 comprises an expandable stent framework 110, and a stent
retaining band 115 which is disposed about the stent the
circumference of the stent 110. Expandable stent framework 110 is a
self-expanding stent which can expand from a reduced diameter
configuration to a fully expanded configuration. Stent retaining
band 115 is disposed about stent framework 110 at an oblique angle
relative to the longitudinal axis 130 of the stent. The invention
is not intended to be limited to the specific oblique angle shown
but rather, is intended to encompass the full range of oblique
angles. The stent may have fewer or more retaining bands than shown
in FIG. 7.
[0041] Another embodiment is shown in FIG. 8. Stent 100 is retained
in a reduced diameter configuration via stent retaining band 115
which is helically disposed about the stent.
[0042] The invention also contemplates the use of retaining
structures such as webs or nets to maintain the stent in a reduced
configuration or less than fully expanded configuration. In general
the web is of a woven or closed cell design that is fused to the
exterior of the stent. The web can be in the shape of a tube that
wraps around individual segments of the stent as shown generally at
415 in FIG. 11a. The web can also wraps around the entire length of
the stent as shown in FIG. 11b. Web 415 is disposed about the
length of stent framework 110. The web desirably has failure points
420 to allow the web to pull apart following failure.
[0043] The web may also be a flat sheet as shown at 415 in FIG.
11c. Web 415 covers a group of cells like a patch. The web patch
may either have failure points such as fatigue points or degradable
portions to release the stent. The web patch may be attached to the
stent struts via a degradable material as shown in FIG. 11d. Web
415 is attached to stent 110 via degradable adhesive 122.
[0044] Another web configuration is shown at 415 in FIG. 11e. Web
415 is in a heaxgonal pattern with fatigue points 420 at the
vertices of the hexagons. Other polygonal configuration may be
substituted.
[0045] As with the retaining segments, the web may be made of a
biodegradable material or may be made of an inert material with
fatigue points. An inert material with biodegradable connecting
material may also be used.
[0046] Although specific web configurations have been shown herein,
those of ordinary skill in the art will understand these to be
exemplary. Other web configurations may be used as well.
[0047] Retaining segments in the form of thin strips may be cut
from sheets of a desired material. Threads of retaining segment may
also be made by extrusion. Fatigue points may be made by varying
the extrusion rate of the thread or by varying the tension on the
thread during extrusion. Tension may also be used to introduce
fatigue points into other types of retaining segments such as
band-shaped retaining segments. Materials such as PTFE will neck
under tension thereby forming a fatigue point. Fatigue points may
also be placed in the material by cutting the material with a laser
or with a punch as illustrated in FIG. 5c. Other methods for
introducing fatigue points in a retaining segment include the spot
application of a laser, of heat, of one or more solvents or other
chemicals such as etchants, or of radiation to weaken or make the
material brittle. Fatigue points may also be introduced into a
retaining segment via the use of a second material in the
segment.
[0048] As shown in some of the figures, the retaining segment is
disposed on the outside of the stent. In situ, the retaining band
is disposed between the vessel wall and the stent thereby
facilitating encapsulation of the retaining band in tissue and
preventing the inert material from being carried away in the
vessel. This is particularly desirable where inert material is used
for the retaining band.
[0049] The retaining segment may be secured to the stent in a
variety of ways. Thread-like and other suitably sized retaining
segments may be woven through the stent and/or tied to the stent or
struts of the stent.
[0050] Alternatively, as illustrated in FIG. 9, a piece of
retaining segment 115a may be placed on an outer surface of a strut
120a and another piece of retaining segment 115b may be placed on
an inner surface of a strut 120b. The two pieces may then be fused
together at 121 by heat, solvent, or adhesive.
[0051] Other methods of attachment include looping a retaining
segment over a strut and securing the segment to itself or the
strut melting with heat or solvent or adhesive. The retaining
segment may also be wrapped around struts or layered around struts
or attached directly to struts and secured thereto by applying heat
or a solvent or other suitable adhesives including bioabsorbable,
biodegradable or erodible compositions such as those disclosed
below. As shown in FIGS. 4c and 10, a biostable material 122 may
also be spot melted to or around a strut 120 and then fused to a
retaining segment 115 made of biodegradable material by heat,
solvent or adhesive. Where a solvent is used, excess solvent may be
evaporated off or otherwise driven off.
[0052] One method of attachment involves providing a mandrel with
degradable dots of material thereon. A stent is then disposed about
the mandrel and a PTFE strip placed over a portion of the stent.
Desired portions of the stent may be cliped so as to retain the
stent in a reduced configuration on the mandrel. Heat is applied to
the stent so as to fuse the dots to the stent and the PTFE. The
stent is then cooled and removed from the mandrel.
[0053] In another method for applying a retaining segment to a
stent, retaining segments are placed in a die which is then
disposed about a stent in a reduced diameter configuration. The
stent is then expanded. Specifically, a circular die at a desired
intermediate stent diameter is provided. The die is provided in two
halves, one of which is die, shown at 500 in FIG. 12, with groves
501 cut into the inner diameter surface for the retaining segment
pattern. Retaining segments of a desired material are placed into
the groves in the die halves and the die halves reassembled. A
mandrel is provided and a nitinol stent is reduced in size onto the
mandrel by cooling the stent below the austenitic-martensitic
transition temperature to make it martensitic. Optionally, the
mandrel may have grooves cut therein in the retaining segment
pattern for placement of a desired material on inner diameter
surface of stent. The reduced stent and mandrel are placed into the
die. The stent is then heated to expand it inside the die.
Adhesives, heat or solvent are then used to fuse the retaining
segments to each other and/or the stent struts, retaining the stent
at the desired diameter.
[0054] In yet another method of applying a retaining segment to a
stent, clips may be used to hold struts or cells in the closed
position in one or more desired portions of the stent. Threads of
retaining segment material are laid across these portions of the
stent and attached to the struts using adhesives, heat, or
solvent.
[0055] Retaining segments may also be secured to a stent by hot
extruding stands of material between confined struts (i.e. struts
in a closed position or reduced diameter position).
[0056] Retaining segments may also be applied by first holding the
stent in a desired reduced diameter configuration via the use of a
retaining die or clamps. Strands of the retaining material may then
be sprayed on the interior of the stent or the exterior of the
stent in the area to be retained.
[0057] FIGS. 13a-13d illustrate a portion of a stent with a
retaining segment prior to and after failure. As shown in FIGS. 13a
and 13c, stent 110 has a retaining segment 115 which retains at
least a portion of the stent in a reduced configuration. Segment
115 is attached to the stent at several points 122. Upon failure of
the retaining segment 115, as shown in FIGS. 13b and 13d, stent 110
expands.
[0058] Suitable biodegradable or bioabsorbable materials for use in
the retaining segments, bands or webs include:
[0059] Poly(L-actide) (PLLA), Poly(D,L-lactide) (PLA),
poly(glycolide) (PGA), poly(L-lactide-co-D,L-lactide) (PLLA/PLA),
poly(L-lactide-co-glyco- lide) (PLLA/PGA),
poly(D,L-lactide-co-glycolide) (PLA/PGA),
poly(glycolide-co-trimethylene carbonate) (PGA/PTMC), polydioxanone
(PDS), Polycaprolactone (PCL), polyhydroxybutyrate (PHBT),
poly(phosphazene) poly(D,L-lactide-co-caprolactone) (PLA/PCL),
poly(glycolide-co-caprolactone) (PGA/PCL), poly(phosphate ester),
poly(DL-lactic acid), poly(glycolic acid).
[0060] PGA and PLLA/PGA are particularly desirable materials. Other
suitable materials include collagen, fibrin, and cellulose.
[0061] More generally, general classes of degradable material
include polyesters, polyamides, polyanhydrides and polyorthoesters.
The latter two are surface erodible types. All of these are
exemplary only.
[0062] As discussed above, the retaining segments may be designed
to last for a predetermined period of time prior to failing. Where
the retaining segment incorporates biodegradable materials, the
lifetime of the segment may be altered by altering the chemical
structure of the biodegradable portion of the segment. Hydrolysis
is the basic reaction for the most biodegradable polymers. The
hydrolytic degradation rate can be altered several thousand-fold by
changing the chemical structure in the polymer backbone. For
example, aliphatic polyanhydrides degrade in a few days while the
aromatic polyanhydrides degrade over a period of a few years.
Polyorthoester is a slow surface eroding material. In the presence
of acid additive, a so-called excipient, it has a faster
degradation rate. In contrast, in the presence of basic substance,
it suppresses degradation. When longer lasting retaining segments
are desired, the aromatic polyanhydride and non-additive
polyorthoester will be preferred segment materials.
[0063] Also, for example, polymers that contain hydrolytically
labile linkages in their backbone can hydrolyze by two different
mechanisms. These are bulk erosion and surface erosion. In a bulk
eroding polymer, the hydrolytic process occurs throughout the
matrix of the polymer whereas in surface erosion the hydrolysis is
only confined to the outer layer of the polymer. Thus, the latter
is especially preferred when longer degradation is desired.
[0064] Specific segment materials include, for example, poly(ortho
esters) such as 50:50 HD/t-CDM
(1,6-Hexanediol-co-trans-Cyclohexanedimethanol) poly(ortho ester)
with 0.2% poly(sebacic anhydride) excipient. Polyanhydrides such as
poly[bis(p-carboxyphenoxy)propane anhydride] (PCPP) and
poly(terephthalic anhydride) (PTA) may also be used.
[0065] If a polyanhydride is selected as the segment material, for
example, its thickness can be selected such as to control
degradation time. Thus, a segment may be tailored to have a
degradation time of a week, a month, two months, or up to six
months, or any other suitable period of time. Segment material of
PGA, for example, may be provided at a selected thickness to
provide a degradation period of about two weeks, for example.
[0066] The retaining segment is desirably designed to fail after a
predetermined period of time following implantation in the bodily
lumen. Failure includes mechanical failure and failure resulting
from the biodegradation of at least a portion of the segment.
Retaining segments which fail after about one week in the lumen are
desirable as are retention segments that fail after about one month
in the lumen. More desirably, the retention segments will last from
about one to about six months following implantation. This allows
the safe application of additional force to the body lumen after
the initial tissue response to the implant and lumen remodeling has
progressed or at a time were additional external force on the body
lumen is expected to increase as in the case of an adjacent growing
tumor.
[0067] The retaining segments may also be designed to fail after
the application of a predetermined amount of force such as by a
balloon.
[0068] The invention also contemplates providing the retaining
segment with a treatment or therapeutic agent. A retaining segment
material that elutes the therapeutic agent can be made by
dissolving or suspending the therapeutic agent(s) and retaining
segment polymer in a solvent. Threads may then be extruded and
films may be sprayed or cast for making strips or webs for use as
retaining segments. The treatment agent may also be provided by any
other suitable techniques including impregnating or coating the
segment with the treatment agent.
[0069] Desirable types of therapeutic agents include
anti-inflammatory agents, anti-proliferative agents, anti-platelet
agents, anti-thrombin agents, anti-oxidant agents, gene therapy
agents and suitable combinations of the above agents. Suitable
treatment agents include drugs such as radiochemicals to irradiate
and prohibit tissue growth and human growth factors including VEGF
(Vascular Endothelial Growth Factor), TGF-beta (Transforming Growth
Factor-beta), IGF (Insulin--like Growth Factor), PDGF
(Platelet--derived Growth Factor) and FGF (Fibroblast Growth
Factor), etc. The drug can be an anticoagulant, e.g. D-Phe-ProArg
chloromethyl ketone. An RGD (Arginine-Glycine-Aspartic Acid)
peptide-containing compound, heparin, antithrombin compounds,
platelet receptor antagonists, antibodies, aspirin, prostaglandin
inhibitors, platelet inhibitors, or antiplatelet peptide may also
be used. The drug can be an inhibitor of vascular cell growth, DNA,
RNA, cholesterol-lowering agents, vasodilating agents. Other
suitable drugs include Taxol.TM. (paclitaxel), 5-Fluorouracil,
Beta-Estradiol. Metabolites and other derivatives of the above
drugs or compounds may also be used. Any treatment agent may be
used, singly or in combination.
[0070] The treatment agent may be impregnated into the retaining
segments or may be provided as a coating on the segments. The
treatment agent may be applied by immersing the segments in the
treatment agent or spraying the agent onto the segments. Suitable
techniques for applying the treatment agent may be found in U.S.
application Ser. No. 08/874190 incorporated herein in its entirety
by reference.
[0071] The invention is also directed to an expandable stent for
implantation in a bodily lumen comprising an expandable stent
framework and a stent retaining segment which is constructed to
fail. As discussed above, the retaining segment may be designed to
fail after a predetermined period of time such as about one week or
about one month. The retaining segment may also be designed to fail
upon the application thereto of a predetermined pressure such as
that provided by a balloon.
[0072] The inventive stents, grafts and stent-grafts may be formed
from any suitable self-expanding and balloon assisted
self-expanding stent or graft known in the art. One such suitable
self-expanding stent is disclosed in commonly assigned U.S.
application Ser. No. 08/511076 filed Aug. 3, 1995, incorporated
herein in its entirety by reference. Another suitable stent is the
Wallstent stent, described in U.S. Pat. Nos. 4,655,771, 4,954,126
and 5,061,275 incorporated herein in their entirety by
reference.
[0073] Suitable grafts may be formed by applying a graft material
such as polytetrafluoroethylene (PTFE) or polyethylene
terephthalate (PET) to the stent and securing it thereto by an
appropriate means such as sutures. Other graft materials including
polyurethane, collagen, silicone, polypropylene and polyolefin may
also be used. The graft material may be used as inner liner for the
stent or as an outer liner for the stent. Other types of
self-expanding grafts may also be used in the practice of the
invention.
[0074] The invention is intended to be used with self-expanding
endoprostheses in general and as such is not limited to coronary
endoprostheses. Non-coronary applications include biliary
prostheses and ureteral prostheses. In particular, the inventive
endoprostheses may prove useful where it is necessary to maintain
the patency of vessels or ducts that are being closed by tumors
including the bile duct and pancreatic tumors.
[0075] The invention provides endoprostheses which may partially
expand or not expand at all during deployment and yet expand at a
later point as a result of failure of a retaining segment.
[0076] In another aspect, the invention is directed to a treatment
method which comprises the steps of implanting an expandable stent
with retaining segments such as those disclosed above, in a bodily
lumen. The retaining segments may be constructed to fail by
biodegradation or erosion or mechanical failure after a
predetermined period of time in the body such as about week or
about a month. In accordance with this treatment method, the stent
is implanted in a less than fully expanded state and expands upon
the passage of a predetermined period of time such as a week, a
month or longer with the failure or dissolution of the retaining
segments. The stent may be delivered to the desired bodily location
using a stent delivery catheter such as those disclosed in U.S.
Pat. Nos. 5,534,007 and 5,571,135 incorporated herein in their
entirety by reference or any other suitable stent delivery
catheter.
[0077] Desirably, the retaining segments will be constructed to
fail after the vessel has sufficiently healed to be able to safely
withstand the increment in force following the failure of the
segments.
[0078] The inventive treatment method also contemplates the
deployment of a stent with retaining segments as disclosed above in
a bodily lumen and the subsequent application of a predetermined
force to the stent to more fully expand the stent. Additional force
may be applied immediately after deployment of the stent as a
`touch-up` or after some suitable period of time following
deployment (i.e. days, weeks or months). The additional force
necessary to break the retention segments may be applied via an
inflatable balloon on a catheter.
[0079] The invention is also directed more generally to an
expandable medical device which is held in a contracted or
partially contracted state by one or more retaining structures such
as the segments described above. Suitable medical device include
stents, stent-grafts, grafts and vena cava filters.
[0080] The above disclosure is intended to be illustrative and not
exhaustive. This description will suggest many variations and
alternatives to one of ordinary skill in this art. All these
alternatives and variations are intended to be included within the
scope of the attached claims. In particular, the invention
contemplates stents with multiple types of retaining segments (e.g.
bands and segments linking only several struts together). As such,
stents which are combinations of those shown in the Figures are
intended. Those familiar with the art may recognize other
equivalents to the specific embodiments described herein which
equivalents are also intended to be encompassed by the claims
attached hereto.
* * * * *